Check if type can be an argument to boost::lexical_cast<string>

Question

I have the following traits class(IsLexCastable) to check if a type can be converted to a string by calling boost::lexical_cast<string>. It erroneously returns true for vector<int>.

#include <iostream>
#include <string>
#include <type_traits>
#include <utility>
#include <vector>

#include <boost/lexical_cast.hpp>

using namespace std;
using namespace boost;

namespace std
{
/// Adding to std since these are going to be part of it in C++14.
template <bool B, typename T = void>
using enable_if_t = typename std::enable_if<B, T>::type;
}

template <typename T, typename = void>
struct IsLexCastable : std::false_type
{
};

template <typename T>
struct IsLexCastable<T, std::enable_if_t<std::is_same<std::string, decltype(boost::lexical_cast<std::string>(std::declval<T>()))>::value> > : std::true_type
{
};

int main()
{
  vector<int> a = {1, 2, 3};
  //  cout << lexical_cast<string>(a) << endl;
  cout << IsLexCastable<decltype(a)>::value << endl;
  return 0;
}

This program prints 1, but lexical_cast<string>(a) results in a compile error. What is the right way to implement IsLexCastable?

(This was compiled with g++48 -std=c++11, and boost 1.55.0.)

Answer 1

Your expression is not sufficient, as the lexical_cast function template takes everything and only reports errors via an internal static_assert. Instead test whether inserting the object into an std::ostream is valid:

template <typename T, typename=void>
struct IsLexCastable : std::false_type {};

// Can be extended to consider std::wostream as well for completeness
template <typename T>
struct IsLexCastable<T,
            decltype(void(std::declval<std::ostream&>() << std::declval<T>()))>
  : std::true_type {};

Demo.
That requirement is called OutputStreamable by the documentation, and the direct one imposed onto the source type.

---

Why did your implementation not work?

decltype only causes the instantiation of the declaration of a function template. The internal static assertion is triggered inside the definition of lexical_cast though, hence it cannot be used in SFINAE.

[temp.inst]/10:

If a function template or a member function template specialization is used in a way that involves overload resolution, a declaration of the specialization is implicitly instantiated (14.8.3).